U.S. Pat. No. Re. 25,770 disclosed an "ebullated" bed process for hydrogenation of residuum hydrocarbons to provide desulfurization or hydrocracking or both. As described therein, the reactants consisting of hydrogen and the residuum are under a controlled upflow velocity that places the particulate catalyst in the reactor in random motion in the liquid environment therein. The resulting contact between the residuum, hydrogen and catalyst results in superior conversion, substantially isothermal conditions, and with a minimum carryover of the catalyst from the reaction zone.
When hydrocracking residuum feed, the ebullated bed process is generally operated below 850.degree.F to avoid excessive coking of catalyst. Such an operation requires reactors of a size sufficient to hold the oil for an average time in the order of one hour to achieve substantial conversion. The carbon lay-down on the catalyst has been limited to a negligible amount, say, less than 0.01 percent of the feed. It has been considered important to maintain this limit, as excessive carbon deposited on the catalyst reduces its activity and requires that the catalyst be withdrawn from the reactor and replaced with new catalyst. Failure to maintain a proper level of catalyst activity will result in unstable reaction products which will ultimately render the process inoperable. One of the great advantages of the ebullated bed process is that it makes possible the addition and replacement of this catalyst. However, a significant cost of operating this process for the hydrocracking of hydrocarbons residuum is the replacement cost of the catalyst.